High temperature, stable SiC device interconnects and packages having low thermal resistance

ABSTRACT

A method of forming packages containing SiC or other semiconductor devices bonded to other components or conductive surfaces utilizing transient liquid phase (TLP) bonding to create high temperature melting point bonds using in situ formed ternary or quaternary mixtures of conductive metals and the devices created using TLP bonds of ternary or quaternary materials. The compositions meet the conflicting requirements of an interconnect or joint that can be exposed to high temperature, and is thermally and electrically conductive, void and creep resistant, corrosion resistant, and reliable upon temperature and power cycling.

This application is directed to a method of forming high-temperatureresistant, mechanically robust, and low thermal and electricalresistance bonds for use in electronic devices, particularly siliconcarbide or gallium nitride-based power electronics, with componentattachments stable under elevated operating temperatures. Theapplication is also directed to a method of attaching solid state lightemitting devices to substrates that leads to a substantial reduction inthe operating temperature and hence increases the light output.

BACKGROUND

Silicon carbide based power electronic systems offer significantlyreduced size, weight and volume, a 65% volume reduction in a motordrive, for example, over other available systems. Because of thereliability of their device junctions, however, currently available SiCbased electronic packages are generally limited to a junction operatingtemperature of less than about 150° C. These devices are typicallyassembled using conventional solder materials and techniques to form anelectrically conductive path between metal contacts on packagesubstrates. At higher temperatures and during temperature cycling, thesesoldered connections tend to be unstable, either from operation of thedevice or the surrounding environment in which the device operates. Thiscauses voids and ultimately debonding in the interconnects, resulting inincreased thermal resistance and unreliable operation of theelectronics. Thermal resistance can be large due to thick (>1 mil) dieattachments and inherently lower thermal conductivity (<40 W/m-K) of theattachment materials. Thermal resistance further increases dramaticallyduring the life of the component due to voiding, which leads toincreased device temperatures and accelerates failures. While brazingcan form more reliable connections, the higher temperatures need to formthe connection (700-1000° C.) can damage the components and devicesbeing assembled. The requirements of high temperature operation, thermaland electrical conductivity, void and creep resistance, and corrosionresistance in devices with joints having low thermal resistance areoften in conflict with each other when solder, binary compositions, orbrazing materials are used.

TLP bonding can also be used to improve the performance oflight-emitting devices. It lowers the device operating temperature dueto lower thermal resistance of the bond, resulting in increased lightoutput. It also results in increased mechanical robustness andmaintenance of light output through the operating lifetime of the devicedue to minimal bond degradation during thermal and power cycling andextended operation.

An approach that has been suggested to provide connections betweencomponents that are stable at higher temperatures is a bonding processreferred to as transient liquid phase (TLP) bonding, which can beaccomplished at lower temperatures (less than about 300° C.). Transientliquid phase bonding starts with the use of a high melting point firstmetal as a contact. A film of a second metal of a much lower meltingpoint is placed on the first metal. The two metals form a system, withthe combination of the metals at specific concentrations having anelevated melting point greater than the melting points of the secondmetal. In U.S. Pat. No. 5,038,996 the metals are then heated to atemperature above the melting point of the mixture, causing aninterdiffusion of the metals, forming a bond. The '996 patent disclosesthe formation of Pb—Sn or Sn—Bi TLP bonded connections between copperleads or surface pads.

U.S. Pat. No. 5,897,341 discloses methods for fabricating semiconductordevices, particularly multi-chip modules, including methods for forminginterconnections between an integrated circuit chip and a substrate bysolid-state diffusion bonding of dissimilar metals. In particular, alayer of cadmium, gallium, nickel, tin or zinc is formed over aluminumor aluminum coated pads by typical metal deposition techniques, such asevaporation, sputtering, CVD, electroplating or electroless plating.Bonding is then accomplished by solid-state diffusion at 100-150° C. tocreate a diffusion bond.

The preceding methods, as well as other methods in the prior art, lackthe reliability and thermal resistance required for wide band gap (SiCand GaN) based power electronics.

SUMMARY

Interconnect and packaging techniques for SiC based electronic devicescapable of operating reliably for extended periods of time when exposedto temperatures up to about 400° C., in particular, continuous operationat 200-400° C., as well as cycling from sub-zero temperatures (e.g. −55°C.) to elevated temperatures (e.g., 400° C.) under high voltage andcurrents and the method of forming these interconnects are disclosed.The process utilizes transient liquid phase (TLP) bonding to attach SiCbased electronic devices to metallized substrates, particularly bondsformed from ternary or quaternary mixtures of Ag, Au, Sn and In oncopper or nickel contacts. Devices with high temperature, thermally andelectrically conductive, void and creep resistant, and corrosionresistant joints with lower thermal resistance that outperform thetypical solder, brazing and binary TLP materials are obtained.

DESCRIPTION OF DRAWINGS

FIG. 1 is a phase diagram showing the melt temperatures of compositionscontaining Sn and Au.

FIG. 2 is a phase diagram showing the melt temperatures of compositionscontaining In and Ag.

FIG. 3 is a schematic diagram showing the procedure to form a TLP bondbetween a SiC device and a substrate.

FIG. 4 is a magnified cross sectional view of a bond formed using TLPbonding between a silver coated SiC device and a silver contact on asubstrate using a Ag—In TLP.

FIG. 5 is a magnified cross sectional view of a bond formed using TLPbonding between a silver coated SiC device and a Ni coated Cu contact ona substrate using a Au—Sn TLP.

FIG. 6 a and 6 b are magnified cross sectional views of a bond formedusing TLP bonding between a coated SiC device and a contact on asubstrate using a Au—Sn—Ag ternary TLP intermediate composition. FIG. 7is a graph comparing the corrosion resistance of a Au—Sn—Ag ternary TLPbond with other metallic bonding systems.

FIG. 8 is a magnified cross sectional view of an example of a bondformed using ternary TLP bonding between a gold coating on a SiC deviceand silver and gold coatings on a copper coated ceramic substrate usinga 80Au-20Sn intermediate with melting point of 280° C., forming aternary Au—Sn—Ag metal bond, with points of elemental analysisindicated.

FIG. 9 is a magnified cross sectional view of a second example of a bondformed using quaternary TLP bonding between a gold coated SiC device anda gold and silver coating on copper contact formed on a substrate usinga 52In-48Sn intermediate with melting point of 118° C., forming aquaternary Au—In—Sn—Ag metal bond, with points of elemental analysisindicated.

FIGS. 10 a, 10 b and 10 c are acoustic scans showing a ternary Au—Sn—AgTLP-packaged SiC diode viewed prior to thermal cycling, after 1000 and1500 thermal

FIGS. 11 a and 11 b are acoustic scans showing a Au—Sn TLP-packagedcycles, respectively, between −20° C. and 300° C.SiC diode as fabricatedand after 1433 thermal cycles between −20° C. and 228° C.

FIG. 12 is an acoustic scan showing a traditional solder packaged SiCdiode after 100 thermal cycles between −20° C. and 228° C.

FIG. 13 is a graph showing the current and voltage characteristics of aternary Au—Sn—Ag TLP-packaged SiC diode tested at various temperaturesup to 200° C., held at 200° C. and for up to 550 hours and then returnedto room temperature.

FIG. 14 is a graph showing the absence of change in current and voltagecharacteristics, when a Au—Sn—Ag ternary TLP-packaged SiC diode wassubjected to continuous operation at 200° C. for up to 2739 total hourswith 2189 hrs of 300V reverse bias applied to the diode.

FIG. 15 is a graph showing the current and voltage characteristics, whena quaternary Au—Sn—In—Ag TLP-packaged SiC FET (Field Effect Transistor)was subjected to 0 volts or 4 volts from gate to source at 300° C. forup to 6375 hours.

FIG. 16 is a graph showing the current and voltage characteristics, whena quaternary Au—Sn—In—Ag TLP-packaged SiC diode was subjected to 300° C.for up to 6375 hours.

FIG. 17 is a schematic representation of a state of the art solderinterconnected power device.

FIG. 18 is a schematic representation of a TLP interconnected powerdevice formed utilizing techniques incorporating features of theinvention.

DESCRIPTION

Techniques and materials are disclosed for forming electricallyconductive attachments between components in wide band gap devices(diodes, transistors, lasers, and integrated circuits). The techniquesdescribed are particularly directed to devices for attachment of siliconcarbide (SiC) devices but are also applicable to devices based ongallium nitride (GaN), aluminum gallium nitride, indium nitride (InN),gallium arsenide (GaAs) and indium phosphide (InP) and/or alloysthereof, and other materials typically used in high power and highfrequency applications. The technique is also applicable to silicondevices, where the low thermal resistance and reliability of the jointcan be utilized. The methods disclosed also allow two sided hightemperature contacts and eliminate the need for conventional wirebonding.

In a preferred method, the bond between components is accomplished byforming a ternary or quaternary transient liquid-phase bond takingadvantage of the stability, reliability, resistance to voiding, and lowthermal resistance of the high temperature compositions formed by thecombination of three or four of In, Sn, Ag and Au. Prior techniquesutilized binary solid solutions or multiple phases of either Ag and Inor Au and Sn to directly bond the devices to various substrates

FIG. 1 shows the phase diagram for combinations of gold (Au) and tin(Sn). Sn melts at 232° C. and Au melts at 1063° C. A eutecticcomposition at 29.3 atomic % tin (or 20 wt. % Sn) has a melting point ofabout 280° C. When a preform with this composition is bought intocontact with Au-rich surfaces on the substrates and the devicemetallization, Sn diffuses rapidly into these surfaces, yielding anoverall composition that is Au rich and contains a mixture of solidsolution of Sn in Au and ξ phase. The re-melting is limited to about498° C. due to low solubility of Sn in Au. Final compositions up to 10atomic percent Sn are practical in the joints. The labels 1, 2 and 3 onthe phase diagram indicate the various stages of evolution of the bond.Alternatively, Au and Sn metals can be bought into contact and when Snis heated above its melting point, preferably above 420° C. to avoid theintermetallic AuSn, the molten Sn will diffuse into the Au to form ahigher melting (498° C.) composition when the Sn concentration isbetween about 6.8 to about 10 atomic percent. At smaller Snconcentrations, a higher re-melting point can be achieved but thisrequires large Au volume during processing. The low solubility of Sn inAu and intermetallic formation with the Cu and Ni layers frequently usedas interconnects severely limits the thermo-mechanical reliability ofthe joint under thermal and power cycling.

FIG. 2 shows the phase diagram for combinations of silver (Ag) andindium (In). Indium, which has a melting point of 157° C., and silver,which has a melting point of 962° C., form a solid solution up to about20 atomic percent In (or 21 weight percent). A bond stable to >900° C.can be obtained while processing at about 300° C. or less. However, thesolid solution of In in Ag has poor corrosion resistance under voltagebias and high temperatures (>200° C.). The metallic material exhibitsmigration and evaporation, which are highly undesirable and limit itsapplication.

FIG. 3 is a schematic representation of a process incorporating featuresof the invention. A SiC device 10 is metallized with Ag or Au 12 or bothAu and Ag using common metal deposition techniques, such as vapordeposition, electrodeposition, etc. A substrate 14, such as AlN isbonded on the top and bottom surface with a conductor, such as copper16; then Ag and/or Au 18 is deposited on the copper, again using commondeposition techniques. The TLP bonding material 20 is then placedbetween the coated SiC device 10 and coated substrate 14, the componentsare brought into contact and clamped together with the TLP bondingmaterial 20 sandwiched between and the assembly 22 is heated. As aresult, the TLP bonding material 20 melts transiently, wetting thedevice and substrate to be joined, diffusing into the coatings andmetallizing until the desired alloy of metals is formed. This alloysolidifies into a ternary or quaternary composition 26, which hasproperties superior to those of solders and binary TLP materials. TheTLP bonding material consists of alloys of Au—Sn and/or In—Sn and may beused in the form of a foil or deposited onto the substrate and/or thedevice. When the assembly 22 is cooled a bonded structure 24 is formed.The solidified interconnect 26 between the SiC device and the substratehas a higher melting point than at least one or more of the metalsmelted to form the bond because the ultimate thermodynamic compositionlies in a region of much higher melting point. The ultimate compositionalso has higher electrical and thermal conductivity than solders or theTLP bonding material 20 since the ultimate composition is rich in Agand/or Au. The combination of higher thermal conductivity and a smallerrequired thickness for the joint 26 provides 3-6 times lower thermalresistance than typical solder or binary TLP materials. The higherre-melting point of the joint 26 gives rise to larger resistance tovoiding during thermal and power cycling, due to larger creepresistance, and therefore the lower thermal resistance is maintainedover the life of the component. It has been shown that the ternary andquaternary TLP materials outperformed both solders and binary TLPcompositions upon thermal cycling of the joint, while retaining theircorrosion resistance.

The process is illustrated by the following examples which describe theformation of two alternative compositions that incorporate the featuresof the invention. However, one skilled in the art will recognize, basedon the teachings herein, other metals, compositions, thicknesses,substrates, temperatures, dimensions, etc can be used to accomplish theformation of a high temperature, thermally and electrically conductive,void and creep resistant, corrosion resistant, and lower thermalresistance TLP bond. As part of the process, a metallic compositionhaving a higher melting point than that of the starting materials isformed. Although the examples and illustrations below are described withthe example of a SiC power device, the technique is equally applicableto other device types including gallium nitride (GaN) and silicon (Si)or combinations of devices—e.g. Si and SiC. In Si based devices, thefollowing technique and composition offers lower thermal resistance,higher void and corrosion resistance compared to soldered interconnects.

EXAMPLE 1

with reference to FIG. 3, a high temperature and electrically conductivejoint is formed in the following manner:

A SiC device with an approximately 2-5 micrometer (μm) layer of Auand/or 25-50 micrometer (μm) of Ag deposited, for example byelectrodeposition or other common coating techniques, and a 6-12.5 milcopper layer bonded to a ceramic substrate, such as AlN or Al₂O₃ isused. Other substrates such as metal matrix composites and metallicsubstrates may also be used.

The copper was polished, 1-2 mil Ag was deposited on the copper and0.08-0.2 mil Au was deposited on Ag, for example by electroplating. Anadhesion layer of Ni and Ti (<1 micrometer) is typically used between Cuand Ag. Ag is deposited first on the copper metallized ceramic substratefollowed by Au.

A low melting (approximately 300° C.) Au—Sn preform, about 0.5-1 milthick, (for example, a film of 80/20 weight % or 71/29 atomic % Au/Sn)was sandwiched between the two substrate and SiC device, mild clampingpressure was applied and the assembly was heated to approximately300-350° C. for about 10-30 min.

The Sn in the melted liquid migrates or diffuses into the Au and Aglayers, rapidly reducing the Sn concentration in the melt, forming ahigher melting Au/Sn/Ag ternary composition between the SiC device andthe copper coated substrate. The assembled structure was then cooled. Ifexposed to an elevated temperature the bond thus formed now melts at atemperature >700° C., thus producing a packaged device which can beoperated at elevated temperatures. The bond thus formed also possessesthe required properties of resistance to voiding during thermal andpower cycling, constant temperature exposure, corrosion resistance, lowthermal resistance and high electrical conductivity.

EXAMPLE 2

An electrically conductive connection can be formed using indium asfollows:

1. Approximately 2-5 micrometer (μm) layer of Au and/or 25-50 micrometer(μm) of Ag deposited, for example by electrodeposition or other commoncoating techniques, on a SiC device and a 6-12.5 mil copper layer bondedto a ceramic substrate, such as AlN or Al₂O₃ is used. Other substratessuch as metal matrix composites and metallic substrates may also beused.

2. A preform of 52 weight % In—48 weight % Sn with melting point of 118°C. or 50 weight % In—50 weight % Sn with melting point of 125° C.approximately 0.5-1.0 mils thick is sandwiched between the metallizedceramic or other substrates and SiC device, mild clamping pressure isapplied and the assembly is heated to a preferred temperature of250-350° C. for about 10-30 min. Higher temperature over and above themelting point of the alloy is preferred to avoid thermodynamicallystable intermetallic phases and to accelerate diffusion. AlternativelyIn or In—Sn alloys may be electro- or vapor deposited on the metallizedsubstrates and/or SiC devices with cooling of the substrate to preventmigration of the evaporated indium or In—Sn into the Au and/or Agcoating during this step. Where indium is deposited, a film of tin isdeposited as well while providing cooling to the substrate. Cooling ofthe substrates and/or SiC devices during deposition, particularly whenthermal and electron-beam evaporation techniques are used is necessaryto prevent diffusion of the evaporated metals due to heating of thesubstrate and/or device. Cooling, preferably provided by a water-cooledfixture to which the substrates and devices are attached, is typicallyturned on after about 2-3 minutes of evaporation. This time periodallows for sufficient migration of the evaporated metal (In and/or Sn)into the Ag and/or Au metallizations on the substrates and devices toimprove adhesion of the subsequent evaporated layers. This techniquedoes not require any performs and may be used to achieve the TLP bond bybringing into contact the substrates and devices with evaporated layers,and heating to 250-350° C. for about 10-30 min.

The In and Sn migrate into the Ag and/or Au layers, thus forming ahigher melting ternary (Ag—In—Sn or Au—In—Sn) or quaternary(Ag—In—Sn—Au) composition between the SiC device and the copper coatedsubstrate. The assembled structure is then cooled. When later exposed toan elevated temperature the ternary or quaternary TLP bond thus formedis stable up to about three times the processing temperature.

While devices stable at higher operating temperatures can be formedusing binary Au/Sn or Ag/In TLP bonds, it has been found that devicesoperable at higher temperatures which are more stable and reliable canbe produced by forming ternary or quaternary compositions, of whichAg—Sn—Au, Ag—In—Au, Ag—In—Sn, Au—In—Sn or Ag—In—Sn—Au are the preferredcombination, because of the higher solubility of Sn or In in Ag than Auand due to the improved resistance to corrosion and void resistance uponthermal and power cycling. These ternary or quaternary bonded devicesalso have higher thermal and electrical conductivity.

FIG. 4 is a cross sectional view of a bond formed in a device with asilver coating on copper using a Ag/In composition. While the indium hashigh solubility in the Ag this combination suffers from severe corrosionunder wet or other environmental conditions because Ag readily ionizesand migrates to the negative electrode.

FIG. 5 is a cross sectional view of a bond formed in a state of the artdevice with a Au—Sn TLP bond using a 80Au-20Sn preform and Ni coated Cuon the ceramic substrate. The SiC device contains a thin Agmetallization but the joint is mostly binary TLP bond of Au—Sncomposition. The utility of such a bond is limited by thethermo-mechanical stability of the low temperature Au—Sn TLP phase inthe joint due to the low solubility of Sn in Au which limits the meltingtemperature to <500° C. and shows creep failure at about 200-250° C.during thermal and power cycling. Additionally, intermetallic formationis observed with complex phases of Ni—Au—Sn where Ni is present andCu—Sn—Au where Cu only is present on the substrate. These intermetallicphases are brittle and further limit the thermo-mechanical reliabilityof the joint. Thermo-mechanical reliability refers to the ability of thejoint to resist voiding and creep and plastic deformation undermechanical loading at operating temperatures imposed on the joint by thedifferences in coefficients of the thermal expansion between themetallized substrates and device. Voiding leads to increase in thermalresistance with time and service that accelerates further voiding bydiffusional or other mechanisms.

FIG. 6 a is a cross sectional view showing the bond between a SiC deviceand a Cu coating on a substrate using a ternary mixture of Au/Sn/Agprocessed at 300° C., said product formed using features of theinvention. FIG. 6 b is an enlarged view of the circled portion of FIG. 6a. No low temperature phases or intermetallic regions were present. Thehigh uniformity of the binding layer that results provides a ternarycomposition with a re-melting point in excess of about 800° C. Thejoints exhibit superior corrosion resistance, thermo-mechanicalreliability, electrical and thermal conductivity.

FIG. 7 is graph illustrating the cationic corrosion resistance of 5different materials used to assemble device packages. The packages wereimmersed in a NaCl+ammonium sulfate solution and bias voltages wereapplied to each package. Measuring the current density provides anindication of the tendency of the joint or interconnect materials toform ions under bias when exposed to an electrolyte. Higher currentimplies rapid corrosion and migration of the cation towards the negativeelectrode. The Ag—In TLP is the worst joint material in terms of itscorrosion resistance. As shown in FIG. 7, the Au—Sn—Ag ternary TLPcomposition has a cationic migration resistance similar to Au—Sn TLP andbetter than pure Au. However, as previously indicated, this ternary TLPbond does not have the operation deficiencies exhibited by the binaryAu—Sn bond. Because of the higher melting point of the ternarycomposition, the assembly has a greater creep and void resistance thanAu—Sn TLP and a superior thermo-mechanical reliability under temperatureor power cycling. Table 1 below compares the corrosion resistance ofvarious joint materials in terms of both anionic and cationicresistance. The ternary Au—Sn—Ag TLP has twice the corrosion resistanceof Au—Sn with far superior thermo-mechanical reliability and resistanceto voiding. The corrosion resistance of ternary TLP is similar to thepopular 96.5Sn-3.5Ag solder, which cannot be reliably used above 200° C.operating temperatures. Corrosion Resistance Material System (MΩ-cm²)Ag—In TLP 0.0725 Au 0.485 Au—Sn TLP 0.554 Ternary Au—Sn—Ag TLP 0.969Sn—Ag Solder 1.09

FIGS. 8 and 9 are cross sectional views through the TLP bond portion oftwo structures formed using features of the invention. FIG. 8 is amagnified cross sectional view showing a bond formed using ternary TLPbonding between a gold coating on a SiC device and silver and goldcoatings on a copper coated ceramic substrate using an 80-20 Au—Snintermediate, forming a ternary Au—Sn—Ag metal bond. Analysis of theconcentrations of the elements Au, Sn and Ag at various points acrossthe bond illustrate that the desired composition of Au—Sn—Ag was formedand the uniformity of the distribution of Sn was achieved.

FIG. 9 is a magnified cross sectional view of a second example of a bondformed using quaternary TLP bonding between a gold coated SiC device andsilver and gold plated on copper contact formed on a ceramic substrateusing a In—Sn preform intermediate, forming a quaternary TLP Ag—Sn—In—Agmetal bond. An elemental analysis across the bond verified thequaternary composition and uniformity of In and Sn in Au and Ag.

FIGS. 10 a, 10 b and 10 c show a comparison of the effect of thermalcycling of a ternary Au—Sn—Ag TLP-packaged SiC diode. The diode consistsof four individual die areas to increase the current rating. Theacoustic scan of the TLP package as fabricated prior to thermal cyclingis shown. Scans after 1000 and 1500 thermal cycles, respectively,between −20° C. and 300° C. were also taken. The acoustic scan of theas-fabricated joint illustrates the excellent bonding between the SiCdevice consisting of four diode areas on a single SiC die. The lateraldimension of the device is about 0.2″. The dark areas indicate voids,grey areas indicate redistribution of the solute (In and Sn) and whiteareas are unchanged compared to the as-fabricated joint. While there aresmall areas of voids, their area fraction is not significant compared tothe total joint area and as demonstrated in the later figures, nochanges in the electrical performance of the device was observed.

In contrast, FIGS. 11 a and 11 b are acoustic scans showing a prior artbinary Au—Sn TLP-packaged SiC diode as fabricated and after 1433 thermalcycles between −20° C. and 228° C. The upper temperature limit of 228°C. was required by the lower thermo-mechanical reliability of the binaryAu—Sn TLP joint as compared to the ternary Au—Sn—Ag joint whichincorporates features of the invention (FIG. 10). The large dark areassurrounding the device edges are voids. Significant destruction of thebond has occurred and the device is no longer usable. A large areafraction (>60%) of the joint (as shown by voids) is observed with onlythe middle portion supporting the current to the device. Even moredamage is shown on an acoustic scan showing a traditional solderpackaged SiC diode (FIG. 12) after only 100 thermal cycles between −20°C. and 228° C. The solder connected device is totally inoperative.

FIG. 13 is a graph showing the current and voltage characteristics of aternary Au—Sn—Ag TLP-packaged SiC diode tested at various temperaturesup to 200° C. The right hand lower line shows the electricalcharacteristics at 200° C. The device was then held at 200° C. for up to550 hours. The electrical characteristics at 200° C. at zero aging andat 550 hours are unchanged. The device was then returned to roomtemperature (23° C.) and the electrical characteristics remeasured. Theyexactly duplicate the 23° C. electrical characteristics measured at thebeginning of the study. The different curves are due to the change inelectron mobility in the SiC diode at different temperatures and are nota result of the joint in the package. A similar result was obtained witha SiC device exposed to 300° C. for 6375 hours.

FIG. 14 is a graph showing a further elevated temperature aging study ofa device prepared according to the teachings herein. A Au—Sn—Ag ternaryTLP-packaged SiC diode was subjected to continuous operation at 200° C.for up to 2739 total hours with 2189 hrs of 300V reverse bias applied tothe diode at 200° C. No change in electrical characteristics oroperational reliability was seen.

FIG. 15 is a graph showing the absence of any significant changes indrain current versus drain-to-source voltage characteristics when aquaternary Au—Sn—In—Ag TLP-packaged SiC Field Effect Transistor (FET)was subjected to 0 volts or 4 volts of gate voltage at 300° C. for up to6,375 hours.

FIG. 16 is a graph showing the absence of any significant change incurrent and voltage characteristics when a quaternary Au—Sn—In—AgTLP-packaged SiC diode was subjected to 300° C. tests for up to 6,375hours.

FIG. 17 is a schematic drawing showing an example of a prior artsoldered power module including a Si IGBT and Si diode. This basicelement is repeated in many power electronic applications. Forcomparison, FIG. 18 shows an example of a two sided, high reliabilityall-SiC power module 50 with 350° C. continuous operability formed usingternary or quaternary TLP bonding 52 incorporating features of theinvention. The device includes a SiC transistor 54 and a SiC Diode 56TLP bonded on both the top and bottom to a appropriately metallizedcopper conductors 58 on ceramic or other substrates 60. The package isthen encapsulated in a non-conductive material 62 capable of continuousoperation at 350° C. and 300-10 KV blocking voltages.

It has now been demonstrated that packages including SiC devicesassembled using ternary or quaternary TLP bonding, particularly usingAu, Sn, In and Ag have superior elevated temperature electricalperformance, stability and reliability when compared to prior artsoldered or binary TLP assembled devices. While the combination of Au,Sn, In and Ag are preferred metals, based on the teachings herein oneskilled in the art will recognize that high temperature ternary andquaternary eutectics can be formed from numerous other combinationswhich include one or-more low temperature melting metals and one or morehigh temperature melting metals which, when combined as taught herein,initially form a melt of the lower temperature melting metal(s) but thealloy or blend after thermodynamic equilibrium having a significantlyhigher melting temperature and meeting other requirements for a powerelectronic application.

1. A process for forming packages of electronic components mounted on asubstrate having a conductive coating, the electronic components andconductive substrate bonded by an electrically conductive metallicmixture, said package operable for extended periods of time attemperatures in excess of about 300° C. and under high voltage loads orcurrent flows comprising: selecting at least a first, second, and thirdconductive metal with different melting temperatures, a first conductivemetal having the highest melting temperature, the second conductivemetal having a lower melting temperature, and the third conductive metalhaving an intermediate melting temperature such that a combination ofsaid conductive metals form the mixture, said mixture having a meltingpoint greater than the melting point of the second conductive metal,forming an electrically conductive coating of one or more of the firstand third metals on the electronic components and the conductivesubstrate, placing a conductive metallic composition which comprises atleast the second conductive metal between the coated electroniccomponent and the coated substrate and clamping said assembly together,heating the clamped assembly to a temperature in excess of the meltingpoint of the conductive metallic composition containing the second metalcausing the metal composition to melt, and maintaining said temperaturefor a sufficient period of time to allow diffusion of the materialcomprising the melted metallic composition into the electricallyconductive coatings on the electronic component and the substrate, andcooling the clamped assembly, the conductive material between thesubstrate and the, electronic component comprising the mixture that hasa melting point greater than the melting point of the second conductivemetal.
 2. The process of claim 1 wherein the first conductive metal isgold, the second conductive metal is tin and the third conductive metalis silver.
 3. The process of claim 1 wherein the first conductive metalis gold, the second conductive metal is indium and the third conductivemetal is silver.
 4. The process of claim 1 wherein the first conductivemetal is gold, the second conductive metal is a combination of tin andindium and the third conductive metal is silver.
 5. The process of claim1 wherein the substrate is coated with copper and the electroniccomponent and the copper coated substrate are coated with gold or silveror gold and silver and the conductive metallic composition placedbetween the coated substrate and the coated electronic componentcomprises one or more layers of metal films or preformed combinations ofmetals comprising Ag, Au, Sn or In alone or in combination such thatupon melting of the Sn, In or preformed combination of one or more ofAg, Au, Sn and In the metals diffuse into adjacent layers of coating andform the mixture.
 6. A process for forming packages of SiC devicesmounted on a nonconductive substrate, the substrate having a coppercoating thereon, the SiC device and the copper coated substrate bondedby an electrically conductive metallic mixture, said package operablefor extended periods of time at temperatures in excess of about 300° C.and under high voltage loads or current flows comprising: forming acoating of gold or silver or gold and silver on the SiC device and thecopper coated substrate, placing a conductive metallic film between thegold or silver or gold and silver coated SiC device and coated substrateand clamping said assembly together, said conductive metallic filmcontaining one or more metals, alone or in combination, having a meltingpoint lower than the melting point of the metallic mixture formed bycombining the one or more metals and the gold or silver or gold andsilver, heating the clamped assembly to a temperature in excess of themelting point of the conductive metallic film, causing the metallic filmto melt, and maintaining said temperature a sufficient period of time toallow diffusion of the material comprising the melted film into theelectrically conductive coatings on the SiC device and the substrate,and cooling the clamped assembly, the conductive material between thesubstrate and the electronic component comprising the eutectic mixturethat has a melting point greater than the melting point of the metal inthe metallic film.
 7. The process of claim 6 wherein the metallic filmis tin.
 8. The process of claim 6 wherein the metallic film is indium.9. The process of claim 6 wherein the metallic film is a combination oftin and indium or tin and gold or indium and silver.
 10. The process ofclaim 6 wherein the metallic film comprises one or more layers of metalor preformed combinations of metals comprising Ag, Au, Sn or In alone orin combination such that upon melting of the Sn, In or preformedcombination of one or more of Ag, Au, Sn and In the metals diffuse intoadjacent layers of Au or Ag forming the eutectic mixture.
 11. A packageof electronic components mounted on a substrate having a conductivecoating, the electronic component and conductive coated substrate bondedby an electrically conductive mixture, said package operable forextended periods of time at temperatures in excess of about 300° C. andunder high voltage loads or current flows wherein the electroniccomponent is a SiC device, the substrate with a conductive coating is acopper coated AlN substrate and the electrically conductive mixturebetween the SiC device and copper coated substrate and bonding the SiCdevice to the copper coated substrate comprises at least a ternary orquaternary mixture having a melting point intermediate between a highermelting component of the mixture and a lower melting component of themixture.
 12. The package of electronic components of claim 11 whereinthe higher melting component of the mixture comprises gold or silver ora combination thereof and the lower melting component of the mixturecomprises tin or indium or a combination thereof.
 13. A method offorming an assembly of two components with an electrically conductiveconnection between a first component and a second component, comprisingthe steps of: selecting a first, second, and third electricallyconductive metal, the first metal having a melting temperature greaterthan the melting points of the second and third metal, the secondelectrically conductive metal having a melting temperature lower thanthe melting temperature of the first metal, and the third electricallyconductive metal having a melting temperature lower than the meltingtemperature of the second metal; depositing a first electricallyconductive layer on a surface of the first component, said first layercomprising at least the first metal; depositing a second electricallyconductive layer on a surface of the second component, the second layercomprising at least the second metal; placing a quantity of the thirdmetal between the first layer and the second layer, with the first andsecond layers between and in contact with the third metal; heating thethird metal to a temperature above its melting temperature andmaintaining the third metal at that temperature for a time sufficient toallow substantially all of the third metal to diffuse into the first andsecond layers to form an electrically conductive mixture between thefirst and second components, said mixture having a melting point greaterthan the melting point of the third metal.
 14. An electricallyconductive assembly, comprising: a first component; a second component;and an electrically conductive metal mixture joining the first andsecond component, said metal composition comprising a first layer on asurface of the first component including at least a first electricallyconductive metal, a second layer on a surface of the second componentincluding at least a second electrically conductive metal and a thirdelectrically conductive metal between and diffused into the first andsecond layer, the first, second, and third metals having differentmelting points with the second metal melting point being intermediatebetween the first metal and third metal melting points, thereby formingan electrically conductive mixture joining the first and secondcomponents said mixture having a melting point greater than the meltingpoint of the third metal.